The present invention relates to the art of bending glass sheets by a widely used technique known as gravity sag bending. In that technique, flat sheets of glass are placed upon bending molds having an outline of the curvature desired for the glass. The glass laden molds are pressed into a tunnel-like lehr wherein the glass sheets are heated until they sag to conform to the curvature of the bending mold. The process is usually carried out in a continuous manner, with glass being loaded onto a progression of molds at the entrance end of a horizontally elongated lehr, the bent glass being unloaded at the opposite end of the lehr, and the molds being continuously conveyed back to the entrance end of the lehr. A typical gravity sag bending opeation may be seen in U.S. Pat. No. 2,794,300 to Golightly.
Sag bending molds are usually made of metal, which somtimes causes problems in a bending operation. Metals have heat capacities considerably greater than glass, so that when a metal mold carrying a piece of glass enters the lehr, the metal absorbs more thermal energy and increases its temperature more slowly than the glass. At points of contact between the glass and the metal mold, this temperature difference causes heat to be conducted from the glass to the metal mold, thereby rendering the thermal condition of the glass non-uniform. Non-uniform heating can produce stresses within the glass which cause breakage of the glass while it is in the lehr or shortly after it has left the lehr, or they may create latent weaknesses in the bent product. It has been known in the art to attack this problem by preheating the bending molds in order to give the temperature of the mold a "headstart" before entering the lehr. This was accomplished by heating means, such as gas burners, located in or near the glass loading station at the entrance end of the lehr. It was also common practice in the art, when starting a new production run, to first send the cold molds through the lehr without glass at least once in order to preheat the molds so as to reduce the thermal differences beteen the metal molds and the glass sheets in the lehr which may arise later. For the sake of fuel efficiency, it would be desirable if both of these practices could be eliminated.